Vacuum fluorescent displays are currently used as the display device in many common home digital clocks and the like. In addition to the common blue-green clock display, a variety of colored displays and high resolution graphic displays are offered as products.
The basic operation of a vacuum fluorescent display involves the emission of electrons from a low work function thermionic filament cathode, the acceleration of the thermionically emitted electrons by an electric field and stimulation of a phosphor anode by the impacting electrons resulting in cathodoluminescent light generation in the phosphor.
In the vacuum fluorescent display, the filament cathode to phosphor anode voltage determines whether the phosphor emits light and the intensity of the emitted light. The vacuum fluorescent display is similar to the common cathode ray tube found in televisions and monitors, except that the cathode ray tube uses electromagnetics to direct electrons to the addressed pixel while the vacuum fluorescent display requires that illuminated pixels be individually biased. Some vacuum fluorescent displays utilize mesh grids to "gate" the electron flow to cathode areas, thereby, the mesh grid may be used to select which pixels to light. By using an array of filament cathodes and biasing individual pixels in combination with grid structures, a matrix addressed flat panel display results. In contrast, the cathode ray tube requires a large depth to allow for electron beam deflection.
In vacuum fluorescent displays, typical filament cathodes are tungsten wire coated with barium and strontium compounds that allow sufficient electron emission at approximately 10 volts bias at 600.degree. C. filament temperature. The common phosphor used at this low bias voltage is ZnO:Zn, which gives a blue-green cathodoluminescent light emission.
A common 3/4" by 3" clock module draws approximately 300 mW to bias the filament (for heating). Further, because of the hot filaments the spacing between the hot filaments and the phosphor is substantial, currently the thickness is approximately 1/4".
The above described and related patent application discloses a single substrate, vacuum fluorescent display incorporating diode light generating devices. One drawback of the diode light generating devices is the fact that the distance between the emitter and the anode is limited by operating voltages. That is, a close spacing must be maintained between the phosphor at the anode and the diamond material at the emitter. The spacing between the anode and the emitter and the applied bias set the extraction field which then determines the electron emission current from the emitter. Because the described spacing is between the phosphor and the diamond emitter, much care must be used in depositing the phosphor.
Many phosphors are prepared at high temperatures as powders. Typical powder phosphor based devices utilize inexpensive substrates (glass) that would not stand up to the temperatures needed to fabricate efficient phosphors. Further, the use of phosphor powders allows the manufacturing process to sequentially deposit and pattern different phosphor powder, which allows the fabrication of color displays. Phosphor powder particle sizes are in the several micron range. However, the spacing between the anode and the emitter generally utilized in the diode device of the above described patent application is about 1.+-.0.1 micron. Generally, the light emitting devices in the above described patent application are limited to using thin film phosphors or very fine grained electrophoretically deposited powder phosphors. This limitation increases the cost and complexity of fabrication.
Thus, a single substrate vacuum fluorescent display in which simple and inexpensive phosphor powders can be utilized as the light emitting layer without requiring special fabrication techniques is highly desirable.
Accordingly, it is a purpose of the present invention to provide a new and improved single substrate vacuum fluorescent display incorporating triode light emitting devices.
It is a further purpose of the present invention to provide a new and improved single substrate vacuum fluorescent display incorporating triode light emitting devices in which the spacing between an emitter and a phosphor carrying anode is not critical to the operation.
It is a still further purpose of the present invention to provide a new and improved single substrate vacuum fluorescent display incorporating triode light emitting devices which are simple and relatively inexpensive to manufacture.
It is yet another purpose of the present invention to provide a new and improved single substrate vacuum fluorescent display incorporating triode light emitting devices in which inexpensive phosphor powders can be utilized.
It is still another purpose of the present invention to provide a new and improved single substrate vacuum fluorescent display incorporating triode light emitting devices in which inexpensive phosphor powders can be simply and easily incorporated into the anode structure.